Method of investment casting



Jan. 28, 1969 c. H. WA' I'TS E AL 7 METHOD OF INVESTMENT CASTING FiledMay 4, 1966 W J 3 1.. J

INVENTORS CLAUDE H. WATTS QO/SEQT 4. HORTON n'rTo/awsm.

United States Patent 12 Claims ABSTRACT OF THE DISCLOSURE In the art ofinvestment casting, a process comprising the steps of forming-a set-upincluding sprue means, divider means which projects from the outersurface of the sprue means around its perimeter and is located betweenthe ends of the sprue means, and a plurality of laterally extendingpatterns attached to the sprue means on each side of the divider means,forming a refractory mold wall around the set-up at each side of thedivider means, removing the set-up from the mold wall to provideseparate refractory molds each having a through passage and a pluralityof pattern cavities gated into the passage, and closing one end of eachmold passage so that molten metal can be cast into the molds.

This invention relates generally to the art of investment casting, andmore particularly to ceramic shell molding techniques of precisioncasting wherein shell molds suitable for casting metal are prepared bybuilding up layers of refractory material around disposable patternswhich are subsequently removed from the molds.

Ceramic shell molds are prepared using patterns which are replicas ofthe parts to be cast, including the necessary gates and risers, andwhich are formed of an expendable material, such as wax or a suitablesynthetic resin or blends of wax and resin. These patterns are attachedto a central sprue member to form what is known as a setup or tree. Theformation of a shell mold around the tree or set-up is generallyaccomplished by applying a refractory slurrycoating of controlledviscosity followed by directional draining to coat the patternscompletely. After draining excess slurry from the set-up, the slurrycoating is sanded or stuccoed while wet with coarser refractorymaterials. The result is a layer of ceramic material having refractoryparticles embedded in the surface. This layer is hardened, as by forcedair drying at room temperature. After the first ceramic layer issufficiently hard and dry, the steps of coating, draining, stuccoing anddrying are repeated until a refractory shell having a sufficientthickness to resist the stresses occurring in subsequent operations hasbeen built up around the set-up. In a subsequent pattern removaloperation, the patterns are destroyed and the shell mold is prepared forthe casting operation.

A recent development of significant importance in ceramic shell moldingand investment molding techniques involves the concept ofinitiallyforming the one-piece refractory mold so that it has an open-ended,through axial passage and a plurality of pattern cavities gated into thepassage. A core may be assembled in the mold passage after the patternremoval operation in order to form a tubular sprue and thereby obtainthe advantages of directional solidification, a low ratio of gatingmetal to casting metal and improved metallurgical characteristics of thecastings.

The formation of mold shells with through passages results in manyeconomies and substantial improvements in all phases of ceramic shellmolding operations, including the removal of the patterns. It ispossible to cast more parts in each individual shell mold, and the moldscan be made quicker and with fewer handling problems 3,424,227 PatentedJan. 28, 1969 than in the past. Because of the through passage in themold, the patterns can be destroyed without cracking the relatively thinwalls of shell molds and in a shorter time than when processingconventional shells. For example, when the patterns are formed of a heatexpendable material, the mold can be quickly heated both internally andexternally. The internal heating of the mold affords almostinstantaneous relief for each pattern cavity gated into the open passageand thus prevents mold cracking due to detrimental pressures created byexpansion of the pattern material during heating. The open ended passageformation also facilitates destruction of the patterns by solventremoval techniques and by the use of steam and pressure.

The purpose of the present invention is to provide a new investmentmolding process whereby a plurality of refractory molds each having athrough passage and. a plurality of pattern cavities gated into thepassage are formed simultaneously using a single set-up or tree. The newprocess is particularly suited to ceramic shell molding techniques andmakes it possible to obtain all of the economies of shells with throughpassages when producing small as well as large production orders.

According to one aspect of the present invention, there is provided aprocess comprising the step of forming a setup including sprue means, aplurality of laterally extending patterns attached to the sprue meansand a divider portion which projects laterally from the sprue meansbetween its ends, the step of forming a refractory mold Wall around theset-up at each side of the divider portion, the step of removing theset-up from within the mold wall to provide separate refractory moldseach having a through passage and a plurality of pattern cavities gatedinto the passage, and the step of closing one end of each mold passageso that molten metal can be cast into the molds.

According to a more specific embodiment of the invention, there isprovided a process of ceramic shell molding comprising the step offorming a set-up including a cylindrical sprue means, a divider portionwhich projects circumferentially from the sprue means between its endsand a plurality of laterally extending patterns attached to the spruemeans, the step of forming ceramic shell sections around the set-up byapplying refractory slurry coatings to the set-up except for the outerperipheral surface of the divider portion, stuccoing the slurry coatingswith refractory particles and drying the stuccoed coatings, the step ofremoving the sprue means from within the shell sections to provideindividual ceramic shell molds each having an open ended cylindricalpassage therethrough, the step of destroying the patterns by circulatinga pattern removal medium through the mold passages to form patterncavities gated into the passages, and the steps of closing one end ofeach mold passage and placing a core in each passage in spaced relationto the inner mold surface.

The central sprue means which forms a part of the set-up used in the newprocess may comprise a cylinder, the divider portion being formed byproviding a ring around the cylinder between its ends. Alternatively, itis contemplated that the sprue means of the set-up may comprise aplurality of axially aligned cylinders, the ends of which are separatedby a circumferentially projecting disc or plate which serves as thedivider portion. In the specific embodiment of the invention hereinafterdescribed in detail, the sprue cylin'der may comprise a tube, acorrugated cardboard sleeve around the tube and a wax coating on theoutside of the sleeve.

The new process of forming a plurality of refractory molds by use of asingle set-up has many advantages. One important advantage is that theproduction of a single set-up for a plurality of molds is quicker andmore economical than the conventional practice of making a separateset-up for each mold.

Another important advantage is that the time, equipment and handlingproblems heretofore involved in the production of ceramic shell moldsare materially decreased. In the conventional ceramic shell moldingprocess, each mold was made using a separate pattern tree or set-up andthese set-ups were individually handled, either manually or by means ofsuitable apparatus, in the dipping, =draining, stuccoing and dryingoperations. Since the process of the present invention makes it possibleto handle only a single set-up in the shell-forming operationssimultaneously to produce several molds, the mold production rates areincreased.

As described above, the formation of a shell mold having a throughpassage is economical and a set-up of many patterns can be processed toform the mold. The practice of the present invention makes it possibleto obtain the same economies when casting small production orders eachof which may require different patterns H and a ditferent casting metal.Patterns for the different orders can be attached to a sprue to form asingle set-up and the set-up processed to form a plurality of shells.After the set-up has been destroyed, these shells can be separately castwith the particular metal required for each order.

Other advantages and a fuller understanding of the invention will be hadfrom the following detailed description when taken in conjunction withthe accompanying drawings.

In the drawings:

FIGURE 1 is a cross-sectional view of an assembly including a preferredset-up or tree used in the process of this invention;

FIGURE 2 is a perspective view of a divider ring assembly used inconjunction with the set-up of FIG. 1;

FIGURE 3 is a perspective view showing. a ceramic shell mold formedaround the set-up of FIG. 1;

FIGURE 4 is a schematic, vertical cross-sectional view showing a ceramicshell mold and core assembly in an apparatus suitable for holding themold assembly in position for a casting operation.

Referring now to the drawings, and to FIG. 1 in particular, theillustrated pattern assembly includes a setup or tree which is generallydesignated by reference numeral 10. The set-up comprises a central spruemember 11 and two groups 12 and 13 of laterally extending patternsattached to the sprue member. The patterns, which are replicas of theparts to be cast and include the necessary gates and risers, are formedof an expendable material, such as wax or a synthetic resin or a wax andsynthetic resin composition. The two groups of patterns 12 and 13 areshown as having a different shape in order clearly to illustrate thatmolds for different parts can be made by use of a single set-up. It isto be understood that more than two groups of patterns can be attachedto the sprue member and that all of the patterns can be identical.

Preferably, the central sprue member 11 is in the form of a hollowcylinder and is comprised of a tube 14 made of cardboard or otherrelatively stiff material and a corrugated cardboard sleeve 15 whichsurrounds the tube 14. As shown, the sleeve 15 is axially corrugatedonly in its inner surface while the outer wall surface of the sleeve issmooth. This preferred structure of the sleeve 15 is such that the tube14 can be easily removed from the sleeve after the shell formingoperations. The smooth outer cylindrical wall of the sleeve 15 isprovided with a thin coating 16 of a low melting point wax. The wax canbe coated on the sleeve 15 by rotating the tube and sleeve assembly on ahorizontal axis in a molten wax bath. The thickness of the wax coating16 which is formed is sufficient to permit the gate ends of the patterns12 and 13 to be secured to the sprue member 11 by locally heating thewax coating and embedding the gate ends in the softened wax. Preferably,the thickness of the wax coating is in a range from about of an inch toabout of an inch.

In alternate constructions, the sprue member 11 may be formed by aninjection molded or extruded tube of wax or synthetic resin. The hollow,open-ended sprue member 11 also may be for-med by a wax coated metaltube or the like.

In accordance with the present invention, the two groups of patterns 12and 13 are separated by a divider portion 19 which forms part of theset-up it). As will become more apparent from the following description,the divider portion 19 facilitates the formation of separate refractoryshells around the two groups of patterns. While only one divider portionis shown in FIG. 1, it is to be understood that a plurality of axiallyspaced dividers may be provided around the sprue member 11 in order toform as many separate shell molds as may be desired.

The illustrated divider portion 19 is in the form of a ring whichprojects radially from the sprue member 11 a distance which should be atleast equal to the wall thickness of the mold to be formed. The wallthickness of a typical shell mold is in the range of from about A; of aninch to about A of an inch. As best shown in FIG. 2, the preferreddivider ring 19 may consist of two segments 20 and 21. Each segment isconveniently provided with a tongue 22 at one end and a complementalslot 23 at the other end. In use the ring segments 20 and 21 are placedaround the outside of the wax coated sleeve 15 in the desired positionand the tongue 22 of one segment is fitted into the slot 23 of the othersegment. The tongues 22 can be secured in the slots 23 in any suitablemanner, such as by cement or, when the segments are formed of plastic,by softening the tongues with a solvent so that they are welded in theslots.

In order to facilitate handling of the set-up 10, a disc 24 is fittedinto each open end of the sprue member 11. A rod 25 extends axiallythrough the set-up 10 and the discs 24 are secured at 26 to the rod byany suitable means. The rod 25 may be provided with a handle (not shown)at one end or with a drive gear (also not shown), so that the set-up canbe rotated during the shell-forming operations. The assembly shown inFIG. 1 is completed by annular plates 27 which are engaged against theends of the set-up and are secured to the discs 24, such as by screws28. The plates 27 project circumferentially beyond the sprue member 11 asufiicient distance to prevent the shell forming material from beingbuilt up around the ends of the set-up.

In carrying out the shell forming operations, the set-up 10 is coated adesired number of times with a refractory slurry in accordance withconventional practice. During application of the slurry coatings, theslurry may be removed from the outer peripheral surface of the dividerring 19 in order to form a separate coating at each side of the divider.After each application of slurry, the setup 10 is directionally drainedto remove excess slurry and the wet coating is stuccoed with refractoryparticles. Each stuccoed coating is hardened, as by forced air drying,to form a refractory layer around the set-up 10. This sequence ofoperations is repeated as many times as is necessary to build up a shellwall having the thickness and strength required for the particularcasting operation. As noted above, the usual wall thickness of a shellmold is on thehorder of from about /8 of an inch to about A of an incAfter the shell forming operations have been completed, the assembly ofthe tube 14, the discs 24, the rod 25 and the end plates 27 are removedfrom the invested set-up. Referring to FIG. 3, it will be seen that themold wall formed around the sprue member 11 is circumferentiallydisrupted by the divider portion 19 to define two refractory shells 35and 36. When the corrugated sleeve 15 is subsequently stripped from theinside surfaces of the shell sections, the sections separate to provideindividual molds. Each mold is then subjected to a pattern removaloperation, such as by placing the molds in a furnace or an autoclave, todestroy the patterns 12 and 13.

Reference is now made to FIG. 4 which illustrates the final stage in theprocess of this invention. As here shown, the sprue 11 has beencompletely removed and the pattern material destroyed to form patterncavities 37 in the shell 35. The inside surface 39 of the shell 35defines a cylindrical passage 40, and the gate ends 38 of the cavities37 open through the surface 39.

A refractory core 41 may be provided within the shell 35 in spacedrelation to the inner shell surface 39 to define a tubular sprue passage42. As shown, the core 41 is a hollow member having a closed dome-shapedend, an opposite open end and a peripheral rim or step 44 around theopen end. The rim or step 44 serves to center the core within the shelland substantially fills the lower end of the sprue passageway 42 so thatlittle or no molten metal can squeeze past the core when the mold iscast.

A suitable apparatus for use in casting the mold and core assembly isshown to be comprised of a vacuum and casting chamber 55. The vacuum andcasting chamber 55 is provided with an outlet pipe 56 which is connectedthrough a suitable valve (not shown) to a vacuum pump or line. Aplatform 58 including a metal plate 59 and a gasket 60 is supportedwithin the chamber 55 by supports 62. The platform 58 has a hole 61formed through the plate 59 and the gasket 60. The core 41 is placed onthe gasket 60 over the hole 61 and the shell 35 is positioned on thegasket around the rim 44 of the core. The chamber 55 is closed by aplate 70 having a center opening which is slightly larger than theoutside diameter of the top portion of the shell 35. An asbestos gasket72 or the like which has an Opening smaller than the inside diameter ofthe shell 35 but which is large enough to pour metal through is placedon the plate 70 so that it covers the top edge of the shell 35.

When the mold and core have been positioned in the manner described, avacuum is drawn within the chamber 55 around the shell 35. At the sametime, a vacuum is also produced within the core 41 because of the hole61 which provides communication between the inside of the chamber 55 andthe inside of the core. The resulting pressure differential on both theshell and the core serves to hold the members in their assembledposition tight against the gasket 60. This differential pressure ismaintained during pouring of the mold to prevent relative movement ofthe core and shell.

The process of this invention will be largely apparent from theforegoing description. In summary, a set-up 10, such as shown in FIG. 1,is formed and is provided with one or more laterally projecting dividerportions 19. The set-up may comprise a single cylindrical sprue member11 which is provided with a ring divider between its ends.Alternatively, a plurality of sprue members 11 may be mounted in axialalignment on the rod 25 and the ends of the sprue members separated byplates such as shown at 27. The plates provided between the ends ofseveral sprue members function in the same manner as the divider ring19. A refractory mold wall is then formed around the set-up at each sideof each divider portion. The divider portion circumferentially disruptsthe mold wall so that when the sprue member or members are removed, theinvested shell separates to provide separate refractory molds eachhaving a through passage. Each refractory mold formed around the spruemember is then subjected to a pattern removal operation. When castingthe shell molds, one end of each mold passage is closed and a core ispreferably assembled within the mold passage in spaced relation to theinner mold wall surface to define a tubular sprue passage.

Many modifications and variations of the process of this invention willbe apparent to those skilled in the art in the light of the foregoingdetailed disclosure. Therefore, it is to be understood that, within thescope of the appended claims, the invention can be practiced otherwisethan as specifically shown and described.

What is claimed is:

1. A process of investment molding comprising the steps of forming aset-up by locating divider means around the perimeter of a sprue meansso that the divider means projects from the outer surface of the spruemeans and is located between the ends of the sprue means, and attachinga plurality of laterally extending patterns to the sprue means on eachside of the divider means, forming a refractory mold wall around theset-up at each side of the divider means, removing the set-up fromwithin the mold wall to provide separate refractory molds each having athrough passage and a plurality of pattern cavities gated into thepassage, and closing one end of each mold passage so that molten metalcan be cast into the molds.

2. A process according to claim 1, wherein the refractory mold wall isformed by the steps of applying a refractory slurry coating to theset-up, stuccoing the slurry coating with refractory particles, anddrying the stuccoed coating.

3. A process according to claim 2, wherein the refractory slurry used toform the molds is removed from the peripheral surface of the dividermeans during application of the slurry to the set-up.

4. A process according to claim 1, including the steps of assembling acore member within the passage of each mold to define sprue passagewaymeans between the core member and the inner mold surface after the stepof removing the set-up.

5. A process according to claim 1, wherein the step of removing theset-up comprises first removing the sprue means and divider means, andthereafter placing the separate molds in a pattern removal medium andallowing the medium to circulate through the mold passages in order todestroy the patterns.

6. A process of ceramic shell molding comprising the steps of forming aset-up including a cylindrical sprue means, a divider portion whichprojects circumferentiatially from the sprue means between its ends anda plurality of laterally extending patterns attached to the sprue means,forming ceramic shell sections, around the set-up 'by applyingrefractory coatings to the set-up except for the outer peripheralsurface of the divider portion, stuccoing the slurry coatings withrefractory particles and drying the stuccoed coatings, removing thesprue means from within the shell sections to provide individual ceramicshell molds each having an open ended cylindrical passage therethrough,destroying the patterns by circulating a pattern removal medium throughthe mold passages to form mold cavities gated into the passages, closingone end of each mold passage and placing a core in each passage todefine spnue passageway means between the core and the inner moldsurface.

7. A process according to claim 6, wherein said sprue means comprise atube, a corrugated cardboard sleeve around the tube and a wax coating onthe outside of the sleeve.

8. A process according to claim 7, wherein the sprue means in a singlecylinder formed by the tube and wax coated sleeve, and wherein thedivider portion is formed by a ring around the outside of the wax coatedsleeve.

9. A process of investment molding comprising the steps of:

(a) forming a set up including sprue means in the form of a cylinder, aplurality of laterally extending patterns attached to the sprue means,and a divider portion which projects laterally from the sprue meansbetween its ends, the divider portion being formed by providing a ringaround the cylindrical sprue means,

(b) forming a refractory mold wall around the set-up at each side of thedivider portion by applying a refractory slurry coating to the set-up,stuccoing the slurry coating with refractory particles, and drying thestuccoed coating,

() removing the set-up from within the mold wall to provide separaterefractory molds each having a through passage and a plurality ofpattern cavities gated into the passage,

(d) and closing one end of each mold passage so that molten metal can becast into the molds.

10. The process of investment casting comprising the steps of:

(a) forming a set-up including sprue means which comprises at least onecylinder including a tube, a sleeve around the tube and a wax coating onthe outside of the sleeve, a plurality of laterally extending patternsattached to the sprue means, and a divider portion which projectslaterally from the sprue means between its ends,

(b) forming a refractory mold Wall around the set-up at each side of thedivider portion by applying a refractory slurry coating to the set-up,stuccoing the slurry coating with refractory particles, and drying thestuccoed coating,

(c) removing the set-up from within the mold wall to provide separaterefractory molds each having a through passage and a plurality ofpattern cavities gated into the passage,

((1) and closing one end of each mold passage so that molten metal canbe cast into molds.

11. A process of investment molding comprising the steps of forming aset-up by locating divider means around the circumference of acylindrical sprue means so that the divider means projects from theouter surface of the sprue means between its ends, and attaching aplurality of laterally extending patterns to the sprue means at eachside of the divider means, forming a continuous refractory mold wallaround the set-up at each side of the divider means, removing the spruemeans from within the having an open-ended cylindrical passagetherethrough,

destroying the patterns by circulating a pattern removal medium throughthe mold passages to form pattern cavities gated into the cylindricalpassages, closing one end of each mold passage and placing a core ineach passage to define sprue pssageway means between the core and theinner mold surface.

12. A process of investment molding comprising the steps of:

(a) forming a set-up by locating divider means around the perimeter of asprue means so that the divider means projects from the outer surface ofthe sprue means, and attaching a plurality of laterally extendingpatterns to the sprue means between both of its ends and the dividermeans,

(b) forming a continuous refractory mold wall around the set-up at eachside of the divider means by the repeated steps of applying a stuccoedslurry coating to the set-up, removing the coating from the outerperiphery of the divider means, and drying the coating, and

(c) removing the set-up from within the mold walls to provide separaterefractory molds each having a through passage and a plurality ofpattern cavities gated into the passage.

References Cited UNITED STATES PATENTS 1,579,743 4/1926 Warlow 134-1293,177,537 4/1965 Horton 16435 3,186,041 6/1965 Horton l64-25 3,283,37611/1966 Hocklin 164l29 I. SPENCER OVERHOLSER, Primary Examiner.

EUGENE MAR, Assistant Examiner.

US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,424,227 January 28, 1969 Claude H. Watts et al.

It is certified that error appears in the above identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 7, line 38, before "having insert mold wall to provide individualrefractory molds each (SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Commissioner of Patents Edward M. Fletcher, Jr.

Attesting Officer

